1. Field of the Invention
This invention relates to zoom lenses of the telephoto type suited to 35 mm cameras, video cameras or electronic still cameras and, more particularly, to zoom lenses having four to six lens units in total, of which certain ones are movable for zooming, and having as high a range as 4 or thereabout with maintenance of a high optical performance throughout the zooming range, while still permitting improvements of the compact form of the entire system.
2. Description of the Related Art
For photographic cameras and video cameras, there have been demanded zoom lenses of high range with high optical performance. Of these, the telephoto type has been proposed in a wide variety of multi-unit zoom lenses in which three or more lens units are movable for zooming.
For example, a 3-unit zoom lens of plus-minus-plus power arrangement in this order from the object side, a 4-unit zoom lens of plus-minus-plus-plus power arrangement, another 4-unit zoom lens of plus-minus-minus-plus power arrangement, a 5-unit zoom lens of plus-minus-plus-minus-plus power arrangement, another 5-unit zoom lens of plus-minus-plus-plus-minus power arrangement, and many others have been proposed, wherein a plurality of lens units are moved to effect zooming.
Since, in these 3-unit, 4-unit and 5-unit zoom lenses, two or more lens units contribute to a variation of the focal length, so that thereby the requirements of minimizing the bulk and size of the entire lens system and of obtaining a desired zoom ratio can be fulfilled at once.
Still another proposal for using six lens units has been made in Japanese Laid-Open Patent Application No. Hei 4-186212. With this 6-unit zoom lens of plus-minus-plus-minus-plus-minus power arrangement in this order from the object side, the zooming range is increased to as high as 10.
In general, for zoom lenses, it is desired not only to improve the compact form of the entire lens system but also to extend the zooming range (increase the zoom ratio). To achieve a great increase of the zooming range, the number of those lens units which contribute to the variation of the focal length may be increased. In addition, the refractive power of every lens unit may be increased to strengthen the zooming effect in some cases. In other cases, the movement of each of those lens units which contribute to the variation of the focal length may be increased.
In the former case, however, to maintain good stability of aberration correction throughout the zooming range, it becomes necessary to increase the number of constituent lenses, giving rise to a difficult problem of improving the compact form of the entire lens system.
In the latter case, to allow full zooming movements, the air separations have to be much increased. This leads to elongation of the physical length of the complete lens. Particularly in a case where the lens units move in complicated relation, the mounting mechanism for the movable lens units becomes very elaborate, giving rise to a difficult problem of improving the compact form of the entire lens system.
Meanwhile, there have been previous proposals for preventing a photographed or picked-up image from shaking. Optical systems having such a capability, or image stabilizing optical systems, are disclosed in, for example, Japanese Laid-Open Patent Application No. Sho 50-80147, Japanese Patent Publication No. Sho 56-21133 and Japanese Laid-Open Patent Application No. Sho 61-223819.
In Japanese Laid-Open Patent Application No. Sho 50-80147, a zoom lens has two afocal zooming sections, wherein letting the angular magnifications of the first and second sections be denoted by M1 and M2, respectively, these sections are made to move in such relation that M.sub.1 =1-/M.sub.2 is mat and at the same time the second zooming section is held in fixed spatial alignment with the original line of sight axis. The shaking of the image is thus corrected to achieve stabilization of the zoom lens against small angle deviation thereof from a desired line of sight.
In Japanese Patent Publication No. Sho 56-21133, vibrations of the optical instrument are sensed by a detector. In response to its output signal, part of the optics deflects to a direction so as to compensate for accidental displacement of the instrument, thus achieving stabilization of an image in space.
In Japanese Laid-Open Patent application No. Sho 61-223819, a photographic system has a variable angle prism of the refraction type arranged at the frontmost position. As the housing containing the photographic system vibrates, this prism varies its apex angle to deflect the image. Thus, the image is stabilized in space for shooting.
Besides these, there are Japanese Patent Publications Nos. Sho 56-34847 and Sho 57-7414, in which an optical member is arranged in part of the photographic system to be held in fixed spatial alignment with the line of sight. As vibrations occur, this optical member and its mating one generate a prism that deflects image light rays. A stabilized image is thus obtained on the focal plane.
Another available method is to utilize an acceleration sensor to detect vibrations of the housing of the photographic system. A lens unit constituting part of the photographic system is made to rotate in the directions perpendicular to an optical axis so that the image is stabilized against jiggles or oscillations at the focal plane.
In general, for the type of photographic system in which one lens unit is made to vibrate in such a way as to prevent the image from shaking, the operating mechanism for image stabilization is required to have capabilities that the tolerable shaking of the image to correct is large enough, that the movement or rotation of that lens unit (shiftable lens unit) is small relative to the oscillation of the image, and that the driving means is small in size and light in weight.
The shiftable lens unit, when decentering, produces decentering coma, decentering astigmatism, decentering chromatic aberrations and decentering curvature of field aberrations. If these aberrations are large, the image is caused to blur, although the shaking of the image is corrected. For example, if large decentering distortion is produced, the image shift in the paraxial zone becomes appreciably different from that in the marginal zone. For this reason, if the paraxial zone alone is taken into consideration in controlling the decentering of the shiftable lens unit to correct the shaking of the image, it is in the marginal zone that a similar phenomenon to the shaking of the image takes place, causing the optical performance to lower objectionably.
In short, for the zoom lenses having the image stabilizing function, it is required that when the shiftable lens unit is moved in orthogonal directions to the optical axis to the decentered position, the amount of decentering aberrations produced is small so the lowering of the optical performance is little and that the required amount of movement of the shiftable lens unit for correcting the equivalent shaking of the image is small, in other words, the so-called decenter sensitivity (ratio of the corrected amount, .DELTA.x, of shaking of the image to the unity of amount of movement .DELTA.H, or (.DELTA.x/.DELTA.H) is large.
According to the prior art, however, the image stabilizing optical systems of the type using an optical member as arranged, regardless of vibrations, to be held in fixed spatial alignment with the line of sight, are not suited to be used in instruments of small size and light weight, because this optical member is difficult to operatively support and because such optical systems are difficult to realize in compact form. Another type of image stabilizing optical system using a variable angular prism as arranged in the frontmost position, though having a merit that, when correcting the shaking, all decentering aberrations except chromatic ones are produced to almost nothing, has problems that the driving members become large in size and that the decentering chromatic aberrations produced from the prism are difficult to correct with ease.
Yet another type of image stabilizing optical systems using one lens unit of the photographic optical system for decentering purposes is considered to be amenable to the technique of minimizing the size of the instrument by proper selection and arrangement of the decentering lens unit. However, there is a difficult problem of simultaneously fulfilling the requirements of correcting well all of the aberrations produced by decentering, namely, decentering coma, decentering astigmatism and decentering curvature of field and of realizing reflection of the sufficiently small amount of decentering movement to a sufficiently great effect of stabilizing the image.